A prior-art image processing system for a printer apparatus is taught in, for example, Japanese Provisional Patent Publication (Kokai) No. 61-103299 concerned with an image processing system incorporated in an image duplicating apparatus. In the image processing system disclosed therein, various control and data signals are supplied from the main or master control unit of the system to the subsidiary system control unit associated with the standard functional modules but also the subsidiary system control unit for each of the optional functional modules provided in the printer apparatus. It may happen in this type of system that a master system control unit fails to receive a reply to the control or data signal once transmitted from the master system control unit to the subsidiary system control unit. In such an occasion, the master system control unit issues a similar control or data signal to the particular subsidiary system control unit a prescribed number of times. If there is no reply received from the subsidiary system control unit after the control or data signal has been sent out the prescribed number of times, the master system control unit determines that the particular subsidiary system control unit is not coupled or not properly coupled to the master system control unit and outputs an error signal.
In the prior-art image processing system of the described type, the subsidiary system control units associated with the optional functional modules are not distinguished from those for the standard functional modules. An optional functional module of a printer or an image duplicating apparatus may be coupled to or uncoupled from the apparatus at the user's option before or after the apparatus is switched in for use. This means that the master system control unit may be able to detect the absence of an optional functional module uncoupled from the apparatus after the apparatus is switched in but could not respond to an optional functional module newly coupled to the apparatus after the apparatus is switched in and the master system control unit is initialized.
Accordingly, it is an important object of the present invention to eliminate such an inconvenience of a prior-art image processing system for a printer apparatus through provision of an improved image processing system which is responsive not only to an optional functional module uncoupled from the apparatus after the apparatus is switched in but to an optional functional module newly coupled to the apparatus after the apparatus is switched in.
Each of the subsidiary system control units in a known image processing system has a memory area in which are to be stored various pieces of data including the data representative of the current status of the functional module which is under the control of the particular control unit. The master system control unit accesses the memory thus incorporated in each of the subsidiary system control units and controls the operation of each subsidiary system control unit on the basis of the data representing the current status of the associated functional module of the apparatus. In this instance, the master system control unit directly accesses the data thus stored in the memory area of each subsidiary system control unit and, for this reason, requires a significantly increased amount of time in accessing the memory areas of all the subsidiary system control units where an increased number of subsidiary system control units are included in the system. The increased amount of time in accessing the memory areas of the subsidiary system control units further result in reduction in the performance efficiency of the master system control unit performing various jobs to control the individual subsidiary system control units.
It is, therefore, another important object of the present invention to provide an improved image processing system enabling the master system control unit to access the memory areas of all the subsidiary system control units more efficiently than the master system control unit used in a prior-art image processing system.
In a prior-art image processing system, the master system control unit is connected to each of the subsidiary system control units through data transmission lines which are proper to the subsidiary system control unit to allow a "report" mode of communication in the system. This type of communication network is useful for achieving a high performance efficiency in the system but requires provision of a more intricate hardware configuration especially where the system incorporates a large number of subsidiary system control units.
In contrast to such a network arrangement between the master system control unit and each of the subsidiary system control units, there is known a system communication network in which the master system control unit is coupled with the individual subsidiary system control units through common data transmission lines or a macrosystem interface bus. This type of communication network allows a "polling" mode of communication in the system and is useful for simplifying the wiring arrangement of the system as a whole. Such a communication network however has a drawback in that the amount of data to be exchanged between the master system control unit and each subsidiary system control unit varies from one subsidiary system control unit to another so that any one or more of the subsidiary system control units may require more time for communication with the master system control unit than the others. The use of more time for communication by a subsidiary system control unit may impair the performance efficiency of the system as a whole since the common communication network must be occupied for an exceptionally long period time by the particular subsidiary system control unit.
Thus, it is still another important object of the present invention to provide an improved image processing system featuring a "hybrid" communication network adapted to alternative selection between the report and polling modes of communication in each of the subsidiary system control units.